1. Field of the Invention
This invention relates to a twist drill having specific chisel edges at its point, more particularly to a twist drill subjected to less drilling thrust, offering good cutting quality in a cutting operation, and having sufficient strength and rigidity.
2. Description of the Prior Art
In conventional twist drills, various types of web thinning have been adopted for the drill points in order to reduce excess drilling thrust and to improve the cutting quality of chisel edges not having thinning cuts. Web thinning involves two types:
(1) diminishing the chisel edge, and
(2) providing a new rake surface to the chisel edge.
In the former, the length of the thinning cut is one-half to three-fourths that of the cutting lip. The cut therefore extends far up the top end of the flute. The average distance of the thinning cut may be one-fourth to one-half of the drill diameter. It is also often recommended to extend the thinning out to the extreme edge in order to change the shape of the chip. In this case, a selectively positive or negative rake is rendered at the full length of the cutting edge.
In the latter as well as most of the former, reconditioning of worn and dull drills takes too much time and skill and it is not so effective. Also, with web thinning, after regrinding the drill point, the troublesome thinning cut operation must be carefully repeated.